The invention pertains to a self-pumping hydropneumatic spring strut with internal level control, in particular used for motor vehicles.
Such spring struts may comprise an oil-filled working cylinder divided into two working spaces by a working piston carried by a hollow piston rod, wherein one working space is connected to a high-pressure chamber and the piston rod is supported against the gas cushion in that high-pressure chamber, which acts as a spring. Further, the spring strut may comprise a piston pump driven by the movements of the piston rod, thus conveying oil from a low-pressure chamber to the working space connected to the high-pressure chamber. The piston pump includes a cylinder formed by the hollow piston rod and a hollow pump rod attached to the working cylinder and carrying a suction valve at its forward end. The pump rod fits into the piston pump cylinder and the bore of the pump rod is connected to the low-pressure chamber. The spring strut further may comprise a control opening, which can be closed as a function of the position of the working piston in the working cylinder, and which connects the working space connected to the high-pressure chamber to a control channel opening out into the low-pressure chamber.
Self-pumping spring struts with internal level control are already known (e.g., DE 196 48 859 C2), in which the piston rod is pushed outward during the operation of the vehicle until it reaches a certain position, and in which a control opening is released to limit the height of the vehicle. At this moment, the control function begins, in that the oil flows via the control opening back to the low-pressure space from the working cylinder under high pressure. The high-pressure space consists of an area filled with oil and a gas cushion, which is sealed off from the oil. Because the spring characteristic curve is slightly progressive because of the high-pressure gas cushion, this system reduces undesirable rocking movements of the vehicle body as the vehicle passes around a curve or during an abrupt change of lanes and in similar driving situations. This undesirable rocking movement also occurs to an increased extent in vehicles which have a relatively high center of gravity. In these vehicles, the problem of rocking is unavoidable.
Self-pumping spring struts are also known (U.S. Pat. Nos. 5,062,616, 5,941,508), in which a second high-pressure chamber is provided in series with the first high-pressure chamber. This second high-pressure chamber can be filled by the movements of the piston rod acting via the piston pump, but an appropriate arrangement of check valves prevents the second high-pressure chamber from emptying itself independently. Through actuation of an appropriate valve, the second high-pressure chamber can be emptied, so that this device can be used for level control, in which the vehicle, which was loaded while stopped, can then be brought up to level. This design does not make it possible to produce different spring characteristic curves.